Main bearing housing of a wind turbine

ABSTRACT

A main bearing housing for supporting a main rotor shaft of a wind turbine, wherein the main bearing housing defines a first end, a second end and a floor region intermediate the first and second ends. The main bearing housing comprises a first bearing arrangement positioned at the first end of the main bearing housing, a second bearing arrangement positioned at the second end of the main bearing housing, wherein the floor region includes a first oil sump positioned at the first bearing arrangement, and a second sump positioned at the second bearing arrangement. Advantageously, the embodiments of the invention provide that the bearings of the main bearing housing are lubricated by a lubrication system that includes sumps positioned at each of the fore and aft bearings of the main rotor shaft. The fore and aft bearings are therefore supplied with oil at suitable lubrication points and are part of the lubrication system that supplies oil to other components in the wind turbine that require oil lubrication, for example the gearbox and/or the generator bearings. The fore and aft bearings of the main bearing housing therefore do not require a separate lubrication system, such as a grease-based system and so the overall lubrication requirements for the nacelle are simplified.

FIELD OF THE INVENTION

The invention relates to a main bearing housing of a wind turbine, andparticularly to lubrication aspects of that housing.

BACKGROUND OF THE INVENTION

In order to capitalise on economies of scale, there has been a generaltrend for wind turbines to be designed with ever larger rotor discdiameters in an effort to increase the energy capture potential, therebylowering the average cost of energy production. This principle hascontributed to year-on-year increases in global installed capacity in aneffort to re-balance the energy generation mix away from non-renewablessuch as oil and gas, towards renewables such as wind and solar.

However, the upward trend of wind turbine size comes with its challengessince the wind turbine towers must be taller, the blades must be longerand stronger, and the nacelles must be larger and heavier. Thecentrepiece of the wind turbine can be considered to be the main rotorshaft, since it carries the hub and rotor blades and harnesses therotational energy generated by the blades so that it can be converted toelectrical energy by the generator. The main rotor shaft and, thus, thebearing arrangement with which it is supported, therefore must beincredibly robust to withstand the huge forces generated during energyproduction.

In one known arrangement, the main rotor shaft extends through a bearingarrangement including a forward bearing that supports the end of theshaft near to the hub, that is the ‘front’ or ‘forward’ end, and a rearbearing support the end of the shaft distal from the hub, that is the‘back’ or ‘rear’ end. The bearings function to ensure that the mainrotor shaft can rotate smoothly and also transfer axial loads andbending moments to a bed-plate or base-frame. This arrangement isgenerally effective at decoupling the gearbox of the wind turbine fromthe axial and bending forces of the main rotor shaft, so that onlytorque is transferred to the gearbox. In order to ensure effectivelubrication of each of the front and rear bearings, in typical knownarrangements the bearings are supplied with a suitable greasingarrangement.

It is against this background that the invention has been devised.

SUMMARY OF THE INVENTION

According to a first aspect, the embodiments of the invention provide amain bearing housing for supporting a main rotor shaft of a windturbine, wherein the main bearing housing defines a first end, a secondend and a floor region intermediate the first and second ends. The mainbearing housing comprises a first bearing arrangement positioned at thefirst end of the main bearing housing, a second bearing arrangementpositioned at the second end of the main bearing housing, wherein thefloor region includes a first oil sump positioned at the first bearingarrangement, and a second sump positioned at the second bearingarrangement.

An advantage of the invention is that the bearings of the main bearinghousing are lubricated by a lubrication system that includes sumpspositioned at each of the fore and aft bearings of the main rotor shaft.The fore and aft bearings are therefore supplied with oil at suitablelubrication points and are part of the lubrication system that suppliesoil to other components in the wind turbine that require oillubrication, for example the gearbox and/or the generator bearings. Thefore and aft bearings of the main bearing housing therefore do notrequire a separate lubrication system, such as a grease-based system andso the overall lubrication requirements for the nacelle are simplified.

Since the first and second oil sumps are located at the bearingarrangement, oil from the bearing arrangement may simply collect in thesumps during use. The oil sumps and bearing arrangements may beconfigured and arranged such that the bearing arrangement is lubricatedby fluid in the sump, in use. This is advantageous in circumstanceswhere circumferential oil supply to the bearing arrangement fails, forexample due to a failure of a lubrication pump. It is also useful as asupplemental lubrication point for bearing arrangement in addition toany oil supplied around the circumference of the bearing arrangement.

The main bearing may further comprise an overflow basin between thefirst and second sumps, and wherein the overflow basin, the first sumpand the second sump each are connected to a fluid drain system. Thefluid drain system may include a first drain passage connected to thefirst sump and a second drain passage connected to the second sump, andwherein the first drain passage and/or the second drain passage isdefined by the main bearing housing. The advantage of such anarrangement is that the oil passages are integral with the main bearinghousing since they are defined by it and not provided by a separatenetwork of external pipes or hoses. By the term ‘integral’, it is meantthat the passages are defined by drillings or bores that are cast-infeatures to the main casting of the main bearing housing. In thisrespect, one or more outlet ports for the first drain passage and thesecond drain passage may be defined by the main bearing housing.Therefore, the outlets may feed directly into appropriate valve-workattached directly to the main bearing housing, rather than requiringaddition hose connections. For this purpose, the fluid drain system mayinclude a drain control valve which is operable selectively to drainfluid from one or both of the first and second oil sumps. The controlvalve may be operated periodically to provide a more effective clean ofthe sump oil content. The drain passage may also include an input from acollector basin.

Optionally, the oil sumps may include one or more baffle plates whichhelp to prevent oil in the sumps from being aerated due to sloshing thatmay occur during movement of the wind turbine during use. At least oneof the one or more baffle plates may be components that are integrallycast with the main bearing housing. Some baffle plates may be castfeatures into the main bearing housing casting. Alternatively, at leastone of the one or more baffle plates may be formed as separatecomponents to the main bearing housing but attached thereto. Thisprovides flexibility as to how the soil sumps are to be configured.

One or both of the oil sumps may include an overflow arrangementconfigured to permit fluid to spill from the respective oil sumps. Theoverflow arrangement may comprise a spill passage configured with aspill inlet at or near the floor pan of the sump and a spill outletlocated in a position that is between and spaced from each of the spillinlet and an upper edge of the sump side wall. Expressed another way,the spill outlet is in a position that is above the spill inlet andbelow the upped edge of the sump side wall. The skilled person wouldunderstand that the terms ‘above’ and ‘below’ should be taken to meanwhen the sump is in its normal orientation in use.

Advantageously, since the spill passage is fed with oil from a positionthat is close to the bottom of the sump, then debris and sediment at thebottom of the sump tends to be entrained with the flow of oil and sotends not to collect at the bottom of the sump. The lubrication systemtherefore is able to clean the oil more effectively because the debrisand sediment is encouraged to circulate around the lubrication system.This is to be contrasted with known sump designs in which oil wouldsimply overflow over the top of the sump side wall when the sump isfull.

Expressed another way, the spill outlet is in a position that is abovethe spill inlet and below the upped edge of the sump side wall. Theskilled person would understand that the terms ‘above’ and ‘below’should be taken to mean when the sump is in its normal orientation inuse.

The spill passage may take various configurations. In one embodiment,the spill passage may be an integral part of the sump side wall. Forexample, the spill passage may be a channel or drilling defined in thematerial of the sump side wall. This may be a particularly convenientway of integrating such a function into the sump.

In one embodiment, the spill passage may be defined by a columnar towerstructure. The tower structure may be integral to the sump side wall ormay be separated from the sump side wall. As a single tower structure,the spill passage provides a single spill point from the sump, and thismay encourage a faster spill flow of oil from the sump which may be moreeffective in entraining larger particles in the oil flow. More than onespill passage may be provided as respective tower structures.

In another embodiment, the spill passage may include a laterallyelongated channel that extends between a spill wall and the sump sidewall. The spill passage may therefore extend across the width of thesump. It is envisaged that such an embodiment may reduce the likelihoodof particles languishing in various regions of the sump floor pan.

The sump may include a drain passage. The drain passage may be separateto or combined with the spill passage. In one embodiment, the drainpassage is connected to the spill passage and extends away therefrom.The drain passage may connect to a return passage which feeds back to anoil tank of a lubrication system of which the main bearing housing formsa part.

It will be appreciated that preferred and/or optional features of thefirst aspect of the invention may be combined with the other aspects ofthe invention. The invention in its various aspects is defined in theindependent claims below and advantageous features are defined in thedependent claims below.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIG. 1 is a view of a wind turbine in which embodiments of the inventionmay be incorporated;

FIG. 2 is a schematic systems level view of the wind turbine of FIG. 1 ;

FIG. 3 is a perspective view of a main bearing housing of the windturbine of FIGS. 1 and 2 ;

FIG. 4 is a schematic section view through the main bearing housing ofFIG. 3 which illustrates embodiments of the invention;

FIG. 5 is a schematic view of a lubrication system that supplieslubrication fluid to the main bearing housing of FIG. 4 ;

FIG. 6 is a plan view depicting the floor region of the main bearinghousing of FIG. 4 ; and

FIGS. 7 and 8 are perspective and side views, respectively, of analternative embodiment of oil sump.

Note that features that are the same or similar in different drawingsare denoted by like reference signs.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

With reference to FIG. 1 , a wind turbine 2 includes a nacelle 4 that issupported on a generally vertical tower 6, which is itself mounted to afoundation 8. The foundation 8 may be on the land, or wholly orpartially underwater. The nacelle 4 houses a number of functionalcomponents, some of which are shown schematically in FIG. 2 , by way ofexample. Such a configuration would be well known to a skilled person.

Here, the nacelle 4 is shown as housing at least in part, the main rotorarrangement 10, a gearbox 12 and a generator 14. For brevity, sometypical components have been omitted from FIG. 2 as they are not centralto this discussion, for example a power converter and yaw drive.However, the presence of such components is implicit and such componentswould be well understood by the skilled reader.

The main rotor arrangement 10 includes a hub 16 coupled to a main rotorshaft 18, which is rotatably supported by a main bearing arrangement 20that is contained within a main bearing housing 22. In this embodiment,the main bearing arrangement 20 comprises a forward bearing arrangement24 and a rear bearing arrangement 26. The hub 16 is connected to aplurality of rotor blades 27, although three blades are typical in aHAWT. The blades 27 are acted on by the wind and therefore torque isapplied by the hub 16 to the main rotor shaft 18 which causes it torotate within a main bearing housing 22. The front and rear bearingarrangements may be referred to as simply ‘bearings’ in this discussionfrom now on.

An input or ‘forward’ portion of the main rotor shaft 18 comprises a hubconnection flange 18 a, by which means the main rotor shaft 18 isconnected to, and driven by, the hub 16. Here the flange 18 a is shownas being connected to a further flange 29 that is associated with thehub 16, such that the two flanges form a coupling between the hub 16 andthe main rotor shaft 18. The flange 18 a can therefore be considered tobe at the hub-connection end of the main rotor shaft 18.

An output portion 18 b of the shaft 18 provides input drive to thegearbox 12. The gearbox 12 steps up the rotational speed of the mainrotor shaft 18 via internal gears (not shown) and drives a high-speedgearbox output shaft 28. The high-speed output shaft 28 in turn drivesthe generator 14, which converts the rotation of the high-speed outputshaft 28 into electricity. The electrical energy generated by thegenerator 14 may then be converted by other components (not shown here)as required before being supplied to the grid, for example, or indeedany electrical consumer.

At this point it should be noted that although in this embodiment twosupport bearings 24, 26 are shown that provide support to the main rotorshaft 18 at forward and rearward positions, arrangements are also knownin which the rearward bearing is omitted and, instead, rear support forthe main rotor shaft 18 may be provided, for instance, by the gearbox12.

The main bearing housing 22 is supported on a base frame 30, which canalso be known as a bed plate. Although not shown here, the base frame 30may be coupled to a yaw drive at the upper part of the wind turbinetower 6 to enable the base frame 30 and, thus, the entire nacelle 4 toyaw with respect to the tower 6 so as to enable the direction of the hub16 to be adjusted with respect to the wind direction.

The base frame 30 is typically a cast component, for example ofiron/steel, and has the function to transfer the main shaft loads fromthe shaft 18, through the bearings 24, 26, the main bearing housing 22,and the base frame 30, and into the wind turbine tower 6.

FIGS. 3, 4 and 6 illustrate a more practical realisation of a mainbearing housing 22 and main rotor shaft 18 for a better understanding ofthe configuration of the relevant components. Note that the overall formand configuration of the main bearing housing 22 is for illustrationpurposes only and is not intended to limit the invention as defined bythe appended claims.

Referring firstly to FIGS. 3 and 4 , the main rotor shaft 18 is taperedalong its length to provide a relatively larger circumference at theforward end 32 of the shaft 18 and a relatively smaller circumference atthe rearward end (not shown) of the shaft 18. It should be noted that itis not essential that the main rotor shaft 18 is tapered.

The forward and rear bearings 24, 26 are situated between the main rotorshaft 18 and main bearing housing 22, at forward and rearward positionsrespectively along the length of the shaft 18. The forward and rearbearings 24, 26 are therefore clamped or sandwiched between the shaft 18and the main bearing housing 22, and enable the shaft 18 to freelyrotate with respect to the main bearing housing 22 during wind turbineoperation, about a rotor axis R that extends through the centre of themain rotor shaft 18.

The exact form and configuration of the front and rear bearings 24,26 isnot material to the invention. It should be noted that as illustrated inschematic form the bearings are shown as plain roller bearings forconvenience. However, in an application such as this, the bearings wouldlikely be configured as conical roller bearings and/or as tapered rollerbearings in order more effectively to deal with axial thrust forces onthe shaft 18. Remaining with FIG. 4 , it will be appreciated that themain bearing housing 22 includes a (region 40 that extends between thefront and rear ends of the main bearing housing 22 and includes variousfunctional features, structures and formations that are involved in thelubrication of the front and rear bearings 24,26. More specifically, afirst oil sump 42 is positioned at the front bearing 24, and a secondoil sump 44 is positioned at the rear bearing 26. The two sumps 42,44are positioned and configured so that lubrication oil that is injected,delivered, or otherwise provided at the front and rear bearingarrangements 24,26 is deposited in the respective oil sumps under theinfluence of gravity so that the oil in the sumps can lubricate thebottom ends of the bearings.

In contrast with known lubrication approaches for main shaft bearings,it will be appreciated that in the illustrated embodiment the mainbearing housing 22 forms part of a fluid-based lubrication system 50,rather than a greased based approach that is typical in the art. Thelubrication system 50 is shown schematically in FIG. 5 to providefurther context for the invention.

As can be seen the lubrication system 50 includes a series of lubricatedsub-systems that are supplied with lubricating fluid from a tank 52.Although various types of lubricating fluid could be used, the genericterm ‘oil’ will be used from now on for simplicity as referring to alubricating fluid in liquid form that is capable of being used forcirculating lubrication, i.e. being stored in a suitable reservoir ortank and from there being repeatedly pumped around a system to variousconsumer units.

The lubricating fluid is drawn from the tank 52 by a pump 54 anddirected along a suitable fluid supply network 56 to the lubricantconsuming sub-systems, which in overview are the main bearing housing22, the gearbox 12 and the generator 14. A fluid return network 57returns fluid from each of the main bearing housing 22, gearbox 12 andgenerator 14 to the tank 52 where it can be recirculated again to thefluid supply network 56 by the pump 54.

Shown here also in the fluid supply network 56 is a filter unit 58 andan oil heater 60 as these are usual components in an oil-basedlubrication system that may be used in a variety of environmentalconditions. It should be noted at this point that the lubrication system50 is shown in FIG. 5 in a simplified form and therefore omits variouscommon components such as check valves, tapping points, pressure gaugesand so on for the purpose of clarity.

The fluid supply network 56 includes feed lines 62 that supplylubricating oil to the main bearing housing 22, the gearbox 12 and thegenerator 14. Two of the feed lines, here labelled 62 a and 62 b,provide lubricating oil to the main bearing housing 22.

More specifically, the first feed line 62 a provides lubricating fluidto the front bearing 24 and the second feed line 62 b provideslubricating fluid to the rear bearing 26. Although not shown here, itwill be appreciated that the two feed lines 62 a and 62 b may supplylubricating oil to one or more delivery nozzles that may be suitablyspaced about the bearing arrangements for optimal oil delivery. The feedlines 62 a and 62 b are also shown on FIG. 4 as directed to the top endof each of the bearing arrangements 24, 26.

Oil that has been injected through the nozzles to the front and rearbearing arrangement 24,26, is thus used for lubrication and subsequentlyflows into the respective oil sumps 42,44 and, from there, into thefluid return network 57. Further details of the oil sumps will bedescribed in more detail in the discussion that follows.

At this point, it will be appreciated that a significant benefitassociated with the above arrangement is that the main bearing housing22 is included in the fluid lubrication system 50 with the gearbox 12and the generator 14. This would not be the case in known systems inwhich typically the gearbox 12 and, optionally, the generator 14 arelubricated with lubricating oil by a pumped system whereas the mainbearing housing 22 is typically lubricated with a different medium,usually grease. This therefore represents a simplification to the knownapproach to lubricating powertrain components of a wind turbine.

The discussion will now focus on the more specific features of the mainbearing housing 22 that function to provide optimal lubrication to thefront and rear bearing arrangements 24,26.

Referring again to FIG. 4 , it has been described above that the floorregion 40 of the main bearing housing 22 includes the first oil sump 42that is located at the first bearing arrangement 24 and the second oilsump 44 that is located at the rear bearing arrangement 26. Each of theoil sumps 42,44 is configured and arranged so that it provides areservoir for lubricating oil at a depth so that that the bottom of thebearing arrangements 24,26 are at least partly bathed or immersed inlubricating oil. This is illustrated in FIG. 4 in which the oil level(L1 and L2, respectively) in each of the first oil sump 42 and thesecond oil sump 44 is indicated as being in line with the rollerelements in the bearings 24,26. It is also illustrated in the plan viewof FIG. 6 in which it can be seen that the oil level in the two sumpsfloods over to the respective bearings. In this discussion the two oilsumps will be referred to collectively for simplicity where they havefeatures in common. Likewise the features common to both oil sumps willuse the same reference numerals.

Each oil sump 42,44 can be considered to comprise a floor pan 61 that issurrounded by a ‘boundary’ or ‘side’ wall 63. The precise configurationof the side wall may vary depending on the shape and configuration ofthe floor region 40 of the main bearing housing 22. For example, if thefloor region 40 has substantial curvature because of the cylindricalshape of the main bearing housing 22, the oil sump may be a feature thatis casted into the floor region 40 of the main bearing housing 22 sothat the side wall is defined in part by the inner surface of the mainbearing housing 22 itself. Another possibility is that the floor region40 is relatively flat, and so the oil sump 42,44 may be a separatecomponent that is placed on and secured to the floor region 40. In sucha situation, therefore, the oil sump 42,44 could be a box shapedcomponent having a floor pan 61 or base which is surrounded by one ormore wall sections that together define a boundary wall of the oil sumpto contain lubricating oil therein.

It should be noted that in the illustrated embodiment of FIGS. 4 and 6 ,the oil sumps 42,44 are defined partly by the laterally curving surfaceof the floor region 40 and partly by end walls. Therefore, the side wall63 of each oil sumps 42,44 is defined in part by an axially inner endwall section 64 and an axially outer end wall section 66. Whereas theaxially inner end wall section 64 is located towards the centre of thefloor region 40 of the main bearing housing 22, the outer end wallsection 66 is located behind the respective bearing arrangement 24,26.

As shown in FIG. 4 , during normal operation the level of oil in theoils sumps 42,44 reaches a depth during operation such that a part ofthe respective bearing arrangements are bathed in oil. However, in orderfor the lubricating oil to be recirculated around the lubricatingsystem, the main bearing housing 22 includes an overflow arrangement 74.As will be discussed in more detail, the overflow arrangement 74comprises a spill passage 76 that is configured with a spill inletopening (hereinafter ‘spill inlet’) 78 at or near the floor pan 70 ofthe sump 42,44 and a spill outlet opening (hereinafter ‘spill outlet’)80 located in a position between the spill inlet 78 and an upper edge 82of the inner side wall section.

The spill outlet 80 is therefore above the spill inlet 78, withreference to the normal orientation of the main bearing housing 22, butbelow the general level of the upper extent of the side wall section 82.The normal orientation can also be considered relative to a depthdirection of the sump, as shown by the reference D′ shown in FIG. 4 .Here, the dimension D′ is denoted by a vertical arrow that is alignedwith the direction of gravity, i.e. acting downwards toward the centreof the earth. Thus, the spill outlet 80 is located above the spill inlet78 when considered in the depth direction, and also the spill outlet 80is located below the sump side wall section 82 when considered in thissame frame of reference. References to the spill outlet 80 being‘between’ and spaced apart from the sump side wall 82 and the spillinlet 78 should also be taken to be along the same reference direction.It is envisaged that a relatively small spacing between the spill outlet80 and the sump side wall section 82 will be sufficient because the sump42,44 will spill from the spill outlet 80 before the sump overflows theside walls. A 1 cm height differential is thought to be acceptable, buta higher flow rate may be achieved by a greater height differential; forexample a height differential between 5 cm and 10 cm. It should be notedthat it is possible for the sump 42,44 to be mounted, in use, so that itis inclined relative to a horizontal datum. In such a circumstance, evenif the spill outlet 80 and the sump side wall section 82 are at the samelevel when considered with reference to the sump itself, the inclinationof the sump means that the spill outlet 80 and the sump side wallsection 82 will actually be at different vertical heights whenconsidered in the direction of gravity. When the sump is inclined inthis way, the surface of the oil in the sump will be a true horizontaland so will flow out of the spill outlet 80 first before it overflowsthe sump side wall section 82.

An advantage of this configuration is that lubricating oil is allowed tospill from the bottom of the oil sump 42,44 where debris and particlessuch as small metal fragments typically collect. The flow of oil fromthe sump 42,44 into the spill passage 76 through the spill inlet 78therefore tends to entrain debris therein and so acts as a cleaningmechanism for the sump 42,44.

The overflow arrangement 74 may be configured in different ways. Oneembodiment is illustrated in FIGS. 4 and 6 , whilst an alternativescheme is depicted in FIGS. 7 and 8 .

Remaining with the embodiment of FIGS. 4 and 6 , it should be noted thatin this illustrated embodiment the inner side wall section 64 is acasted component in common with the main bearing housing 22 and that thespill passage 76 is an integral part of the inner side wall section 64.More particularly it can be seen from FIG. 6 that the spill passage isdefined by a tower structure 84 that extends upwardly from the floorregion 40 of the main bearing housing 22.

In this embodiment, the tower structure 84 is located in approximately amid-position along the lateral extent of the inner side wall section 64,which can clearly be seen in FIG. 6 . This is an elegantly simplesolution, but it will be appreciated that other configurations arepossible. For example, the tower structure 84 may be positioned more tothe left-hand or right-hand side of the inner side wall section 64.Another option is that there may be provided with more than one towerstructure with associated spill passages. Another option is that asingle tower structure with associated spill passage may be fed by morethan one spill inlet.

As can be seen by the inset panel in FIG. 4 , lubricating oil from thebottom or floor pan 70 of the sump 42,44 flows into the spill inlet 78and up through the spill passage 76 due to the pressure of lubricatingoil above it in the sump 42,44. As the sump 42,44 fills with lubricatingoil, following injection into the bearing arrangements by the nozzles,the level of oil in the spill passage 76 increases until such a pointthat the oil passes out through the spill outlet 80. As the spill outlet80 is in a lower position than the inner side wall section 64, the oilin the spill passage 76 exits the spill outlet 80 and leaves the sump42,44.

The oil spills out of the sump 42,44 into an overflow or ‘collector’basin 86 that is located between the two sumps. In this embodiment thecollector basin 86 is a part of the floor region 40 of the main bearinghousing 22.

The main bearing housing 22 also comprises a drain system 90 that isconfigured to drain lubricating oil from the sumps 42,44 and thecollector basin 86.

In the illustrated embodiment, as will be described, the drain system 90comprises a network of fluid passages that are integral to the structureof the main bearing housing 22 in the sense that they are part of thecasting. This is a particularly convenient way to form the drain system90 as it reduces the number of hose connections required to connect themain bearing housing 22 into the lubricating system 50. However, one ormore of the passages may also be embodied as pipes or hoses that areexternal to the main bearing housing 22.

The functionality of the drain system 90 is to provide a permanentlyopen drain for the collector basin 86 but to provide a selective drainfunction for each of the two sumps 42,44. In this way, therefore, thecontent of the sumps 42,44 may be purged occasionally to enable thelubricating oil within the sumps to be recirculated back the tank 52 andthe filter 58.

To this end, each of the two sumps 42,44 includes primary drain passage92 that links the respective sump to the drain system 90. In theillustrated embodiment, the primary drain passage 92 is an extension ofthe spill passage 76, and so is fed from the spill inlet 78. As can beseen, the primary drain passage 92 extends downwardly from the spillpassage 76 and terminates at a respective drain outlet 94 defined in theunderside of the main bearing housing 22.

In addition to the primary drain passage 94, each sump 42,44 is alsoprovided with a secondary drain passage 96. The secondary drain passages96 are configured to collect oil that has overflowed from the axiallyouter end wall section 66 of each sump 42,44 and feed back that overflowto the primary drain passage 92. Therefore, each of the secondary drainpassages 96 are connected to the respective spill outlets 94. In theillustrated embodiment, it will be noted that the secondary drainpassages 96 are connected to the respective drain outlets 94 via aconnection or junction 97 to the primary drain passage 92. A furtherpassage 99 extends from the connection 97 to the drain outlet 94.

Both of the drain outlets 94 are connected to a control valve 98. Thecontrol valve 98 is configured to selectably close or open either oneof, or both of, the drain outlets 94. The outlet of the control valve 98discharges into the fluid return network 57. Therefore, the controlvalve 98 controls the flow of lubricating oil from each of the sumps42,44 through the respective drain outlets and into the fluid returnnetwork 57.

It should be noted that in the illustrated embodiment, the collectorbasin 86 also includes a respective drain passage 100. Here, thecollector drain passage 100 is embodied as an integral passage ordrilling in the main bearing housing 22 that extends downwardly from thecollector basin 86 to terminate at a connector 102. The connector 102provides an interface to the fluid return system 57. Therefore, the oildrain route from the collector basin 86 is not controlled by a valve inthis embodiment. Optionally, the collector drain passage 100 could bevalve-controlled.

In this embodiment, the control valve 98 is a single three-way valve.Although it is envisaged that the same functionality could be achievedby separate two-way valves, a single valve is particularly beneficialsince such a valve tends to be much more compact than the equivalent useof two valves. Also, a single three-way valve only has a single fluidcontrol mechanism, a single fluid connection with the main bearinghousing, and a single electronic connection for control input purposes.Therefore, a single three-way valve is much more cost effective than theuse of individual valves to control the flow from each drain passage. Itis envisaged that in a normal mode of operation, the control valve 98would close communication between the sumps 42,44 and the fluid returnnetwork 57 so that the sumps 42,44 would fill with oil up to apredetermined depth. Then on a periodic basis, the control valve 98would be operated to drain the oil from each of the sumps 42,44. Sumpdrainage could be carried out on one sump at a time or both sumpssimultaneously. It is envisaged that control over the control valve 98would be achieved by the same computer control system that controls theother components of the lubrication system. This could be a separatecontrol system to a main control system for the other wind turbinesub-systems, or the functionality could be integrated into that maincontrol system.

As a further optional enhancement, the sumps 42,44 may include baffleplates 104. As shown in the illustrated embodiment, the baffle plates104 are located in each sump 42,44 and extend vertically from the floorpan 70 so as to divide the volume of the sumps into compartments. Inthis way, the baffle plates 104 function to reduce the sloshing of oilin the sumps as the wind turbine sways during operation.

Upper edges of the baffle plates 104 may extend to a point about theintended maximum oil levels L1,L2 in the sumps which may increaseeffectiveness.

In order to allow oil to circulate in the sumps 42,44 the baffle plates104 may be provided with suitable apertures. These may take the form ofholes or perforations in the baffle plates 104 to allow oil to passthrough. Alternatively or in addition the apertures may be defined at alower edge of the baffle plates such that a gap 106 exists between thefloor pan 70 and the baffle plate 104. Oil in the sump 42,44 istherefore able to flow under the baffle plates 104, but the baffleplates are still effective at preventing oil from excessive sloshing inthe sumps 42,44.

It is envisaged that the some or all of the baffle plates 104 may beintegral components with the main bearing housing 22 in the sense thatthey are part of a single casting. Alternatively, in another embodimentthe baffle plates 104 may be separate components that are affixed to thesumps 42,44 after the main bearing housing 22 has been manufactured.Optionally, the baffle plates 104 may reach edge-to-edge across thewidth of the sump 42,44, as is shown here, but this is not essentiallyand it is envisaged that in other embodiments gaps may be left betweeneither or both lateral edges of the baffle plates 104 and the side wallof the sump 42,44.

An alternative design of sump 110 is shown in FIGS. 7 and 8 , which hasmany similarities with the sumps 42,44 as described above and so couldalso be used in a main bearing housing 22. As such the sump 110 includesa floor pan 112 and an upwardly extending side wall 114. The side wall114 in this arrangement is rectangular in form although it should benoted that this is not essential.

The side wall 114 of the sump 110 includes a first, axially inner, sidewall section 116 and a second, axially outer, side wall section 118,when considered in the orientation the sump 110 would take when situatedin a main bearing housing 22 as described above.

An overflow arrangement 120 is located adjacent the inner side wallsection 116. It will be noted that in contrast to the tower-likestructure of the overflow arrangement 74 of the previous embodiment, theoverflow arrangement 120 in this embodiment includes an elongated spillpassage 122 that extends laterally across the front of the sump.Therefore, the spill passage 122 is defined by a shallow box-likestructure defined by the inner end wall section 116 and a further endwall or ‘spill wall’ 124. The height of the further end wall defines theheight of a spill outlet 128, which is lower than the height of theinner end wall section 116, more particularly and upper edge 117thereof.

A spill inlet 130 is defined at the bottom of the sump 110, between thefloor pan 112 and a lower edge of the inner wall section 116. The flowof oil through the overflow arrangement 120 is therefore very similar tothat of the previous embodiment in that oil flows into the spill passage122 through the spill inlet 130 which is at a very low position in thesump 110. The oil then travels up through the spill passage 122 and outof the spill outlet 128. Note therefore that similarly to the previousillustrated embodiment, the spill outlet 128 is positioned above thespill inlet 130 but below the upper edge of the side wall. Expressedanother way, the spill outlet 128 is located between the upper edge ofthe side wall and the spill inlet 130.

As in the previous embodiment of FIGS. 4 and 6 , the sump 110 of thisembodiment includes baffle plates 132. However, only two baffle plates132 are shown here for clarity. However, as is shown in FIG. 8 , one ofthe baffle plates 132 includes a plurality of apertures or perforations134 to allow oil to pass through it, and the other of the baffle plates132 includes a gap 136 defined between a lower edge 138 of the baffleplate 132 b and the floor pan 112 of the sump 110.

It should be noted that depending on the construction of the sump, theinner end wall section 116 could in effect be defined by a baffle plate132 having an opening towards its bottom edge, or a baffle plate whichdefines a lower gap at the floor pan 112 of the sump 110.

Although the oil sump of FIGS. 7 and 8 has been described in isolation,for the avoidance of doubt it is stated here that the oil sump of thisillustrated embodiment could be used in a main bearing housing 22 likethat in the embodiment of FIGS. 4 to 6 in substitution for theillustrated oil sump configuration described therein.

It should be noted that the discussion above describes various variantsto the illustrated embodiments and modifications that could be made bythe skilled person that are not considered to fall outside of the scopeof the invention as defined by the appended claims.

Other options are possible.

1. A main bearing housing for supporting a main rotor shaft of a windturbine, wherein the main bearing housing defines a first end, a secondend and a floor region intermediate the first and second ends, andcomprises: a first bearing arrangement positioned at the first end ofthe main bearing housing, a second bearing arrangement positioned at thesecond end of the main bearing housing, wherein the floor regionincludes a first oil sump positioned at the first bearing arrangement,and a second sump positioned at the second bearing arrangement.
 2. Themain bearing housing of claim 1, further comprising an overflow basinbetween the first and second sumps, and wherein the overflow basin, thefirst sump and the second sump each are connected to a fluid drainsystem.
 3. The main bearing housing of claim 2, wherein the fluid drainsystem includes a first drain passage connected to the first sump and asecond drain passage connected to the second sump, and wherein the firstdrain passage and/or the second drain passage is defined by the mainbearing housing.
 4. The main bearing housing of claim 3, wherein one ormore outlet ports for the first drain passage and the second drainpassage are defined by the main bearing housing.
 5. The main bearinghousing of claim 2, wherein said fluid drain system includes a draincontrol valve which is operable selectively to drain fluid from one orboth of the first and second oil sumps.
 6. The main bearing housing ofclaim 5, wherein the control valve is coupled directly to the mainbearing housing so as to interface with the one of more outlet ports. 7.The main bearing housing of claim 1, wherein the first sump and/or thesecond sump includes one or more baffle plates.
 8. The main bearinghousing of claim 7, wherein at least one of the one or more baffleplates are integral to the main bearing housing.
 9. The main bearinghousing of claim 7, wherein at least one of the one or more baffleplates are formed as separate components to the main bearing housing butattached thereto.
 10. The main bearing housing of claim 1, wherein thefirst and/or the second sump includes a boundary wall, and a spillpassage to allow lubrication fluid to spill from the respective oil sumpthrough the boundary wall, said spill passage having an inlet openingand a spill opening, wherein the inlet opening is at a lower positionthan the spill opening.
 11. The main bearing housing of claim 10,wherein the inlet opening is located adjacent a floor of the oil sump.12. The main bearing housing of claim 10, wherein the spill opening islocated at an upper portion of the boundary wall.
 13. A wind turbineincluding a main shaft rotatably supported by a main bearing housing asclaimed in claim 1, a gearbox coupled to the main shaft, and alubrication system including a tank for lubrication fluid, a lubricationpump to draw lubrication fluid from the tank, and a fluid pipe networkfor conveying lubrication fluid from the lubrication pump to one or morelubrication points on the main bearing housing and one or morelubrication points on the gearbox.
 14. The wind turbine of claim 13,wherein the lubrication points on the main bearing housing directlubrication fluid to the first bearing arrangement and the secondbearing arrangement.